Measuring and control apparatus



250-201 Au cm MANHNLK FIPBZIZ XR 2,172,064

Sept. 5, 1939. T. R. HARRISON 2,172,064

IBASURING AND CONTROL APPARATUS Filed Dec. 23. 193'! 2 Sheets-Shaet 1 ll Na 3| 5 32 29 b p 5 I I @215 41 I2 27 ag I20 as 18 Lb "L2 l6 l3 FIG 2 l8 FIG 3 a FIG 4 I M FIG 5 M w 3 '6 3 mmvrm n D? I? \g moms R. HARRISON l5 5 sy FIG. 6. FIG.Z '5

4 M ATTORNEYS.

i4 0 U Z U l ChOSS RLFERENCE a -dudp 1939- 'r. R. HARRISON 2,172,064

IBASUBING AND CONTROL APPARATUS Filed Dec. 23, 1937 2 Shaets-Shqet 2 FIG. I3.

INVENTOR. THOMAS R. HARRISON A TTORNEYS.

Patented Sept. 5, 1939 UNITED STATES PATENT oFFlcr:

I 2,173,064 mtssuamo AND. cormrm. mam-roe Application December 2:, 1937. Serial No. 181,309

4 Claims.

The present invention relates to instrumentailties useful in measuring and/or controlling a variable condition, and more particularly to electronic measuring and/or controlling appa- 5 ratus adapted to produce eilects in accordance with the deflection of an element which controls the apparatus and deflects in accordance with variations in a control quantity or in a quantity to be measured, and in which because of the small magnitude of the element deflecting force it is not desirable, nor possible, to have the said efiects directly produced by the deflecting element.

A general object oi the invention is to pro- 1 vide an accurate measuring and/or controlling system which is quick in its action and does not overrun or hunt and is especially adapted to produce the desired recording and/or controlling eilects in response to small deflections of the most sensitive measuring instruments.

A specific object of the invention is to provide a measuring instrument in which an element deflecting in accordance with changes in the value of a quantity to be measured is included in an instrument comprising a single light sensitive L cell and an electronic relay mechanism which is adapted to continuously adjust the recording pen, indicating pointer, or other exhibiting part of the instrument in accordance with the deflections of the deflecting element.

A further object of the invention is to provide an instrument of the kind referred to above in which a single light sensitive device is employed to produce an alternating current potential variable in phase and amplitude in accordance with deflections of a delicate instrument, such as a galvanometer, and which potential may be amplifled and employed to continuously control the selective energization of relatively heavy appa- In accordance with a preferred embodiment oi the invention two sources of fluctuating light which become bright and dark in alternation, one source being bright while the other source is dark, are provided for illuminating a single light sensitive device, and the light sensitive device is selectively exposed to light from one or the other of the sources or shielded from both accordingly as a galvanometer deflects in one direction or the other from a neutral position or assumes said neutral position. fluctuating light falling on the light sensitive device produces a pulsating electromotive force in the latter which may be readily amplified and the amplified electromotive iorce is employed to control the energization of a reversible electrical motor. -As will be clear, the pulsating electromotive force produced inthe light sensitive device when the latter is exposed to light from one of the sources so mentioned will be dimlaced 180 in phase from that produced in the light sensitive device when the latter is exposed to light from the other source, and when the light sensitive device is shielded from both of the sources. the pulsating component of the electromotive force induced therein will be reduced to zero.

As will become apparent, I have provided a system in which a single light sensitive device is capable of controlling the selective operation of a reversible motor without the agency of relays having mechanically moving parts, and since a single light sensitive cell may be employed to produce the desired motor energizing effects, it will be clear the system is not dependent upon the individual characteristics of light sensitive cells. It is noted furthermore that, since the system is dependent upon the derivation of a pulsating electromotive force in the light sensitive cell of one phase or of the opposite phase for producing the desired motor energizing effects, changing cell characteristics will have no effect on the operation of the system.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

0f the drawings:

Fig. l is a diagrammatic view illustrating one embodiment of the invention as adapted for use in measuring and recording the temperature in a furnace;

Figs. 2-7 illustrate in detail the manner in which galvanometer deflection exposes the light responsive eleme ty of Fig. l to one or the other oi the plates of a neon lamp or shields the element from both plates;

Figs. 8-10 illustrate in detail a modification of the arrangement illustrated in detail in Figs. 2-7;

Figs. 11 and 12 ilhistrate two typical damping devices which may be employed with the arrangement of Fig. 1; and

Fig. 13 is a diagrammatic view illustrating the use of my invention in a control system.

In Fig. l of the drawings I have illustrated, 'more or less diagrammatically, a pyrometer for measuringand recording the temperature-of the interior of a furnace i. As illustrated, a thermocouple 2 is arranged in the interior of the jurnace and the terminals of the thermocouple are connected by conductors I and l to the terminals of a null point potentiometric network I which may be of any suitable type such as the Brown potentlometrlc circuit disclosed in my prior Patent 1,898,124, issued February 21, 1933. The moving coil of a galvanometer 8, which is of the reflecting type and equipped with a mirror 1, is connected in the conductor 3. The turning of the galvanometer moving coil is desirably restricted by mechanical stops (not shown).

The potentiometric circuit 5 is of a well-known type, and it is suflicient for the present purposes to note that the potentiometric circuit includes a circuit branch including the thermocouple 2, an opposing circuit branch including a source of known potential such as a battery 8, and resistors 9, a variable portion of which may be connected into the opposed branches by means of a sliding contact I whereby the respective effects of the variable and known sources are made equal and opposite, and the galvanometer moving coil is thus rendered undefiected when the circuit is balanced for a given value of the E. M. F. of the thermocouple 2 with the contact II in a corresponding position along resistors O. The position of contact I0 is then a measure of the value of the thermocouple E. M. F. and may then serve as a measure of the temperature to which it is exposed.

In the embodiment illustrated in Fig. 1, my invention is specifically concerned with the means by which the contact I0 is adjusted back and forth along resistors 9 in response to a galvanometer deflection, and, as shown, includes an electronic amplifier II and a reversible electrical motor l2, the rotation and direction of rotation of which is adapted to be selectively controlled by amplifier H. Motor I2 is connected in any convenient manner to contact III for adjusting the latter along resistors 9, and as will be clear. amplifier H is employed for selectively controlling the motor rotation in response to a feeble pulsating voltage which is derived, in a manner to be presently explained, as a result of galvanometer deflection.

A photoelectric cell H, which is preferably mounted in a casing (not shown) so as to be shielded from all external light except that permitted to pass through a narrow slot I in a shield l which may or may not comprise part of the casing, as desired, is connected to the input terminals of amplifier H and is adapted to be selectively exposed to one or the other of two beams of light which emanate from respective plates l1 and it of a neon lamp l6. Neon lamp I6 includes two plates l1 and I8 which are preferably positioned in the same plane and may desirably be in the form of half moons. One plate is connected to the alternating current supply conductor L and the other plate is connected to the alternating current supply conductor L.

Plates I1 and I8 light up in rapid alternation at a frequency corresponding to that of the alternating current supply conductors, one plate being bright while the other is dark. The light beams emanating from each plate are reflected towards photoelectric cell II by mirror 1 which is carried by galvanometer I, and when the latter is deflected in one direction or the other from its neutral position, one or the other of the beams is reflected through slot ll of shield I5 onto-cell l3.

The manner in which the deflection of galvanometer 6 from its neutral position operates to derive a pulsating voltage in photoelectric cell I3 is best described in connection with Figs. 2-7. As illustrated in Figs. 2 and 3, the galvanometer mirror is indicated in its undefiected position.

In Figs. 4 and 5 mirror I is indicated as deflected in a counterclockwise direction from its undefiected position, and in Figs. 6 and 7, is indicated as deflected in a clockwise direction from its undefiected position.

Referring to Fig. 2, in which plate 11 is shown illuminated, it will be noted that the light beam emanating therefrom is reflected from mirror 1 onto shield I! to the right of slot ll so that no light impinges on photoelectric cell It. In Fig. 3, in which plate I8 is shown illuminated, it will be noted that the light beam therefrom is reflected by mirror 1 onto shield l5 to the left of slot l4 so that in this case also no light impinges on photoelectric cell It. Thus, when mirror 'I is in its undefiected position, photoelectric cell It remains dark during the alternate illumination of neon lamp plates l1 and It whereby the current conducted by the cell remains a steady predetermined value.

In Fig. 4, as indicated by the arrow, mirror 1 is deflected counterclockwise from its undefiected position to a position in which light emanating from neorilwmll is reflected by mirror 1- through 5 0 ll onto photoelectric .cell lj. When plate It is illuminatedand plate I! is dark, however, as illustrated in Fig. 5,,light is reflected from plate It by mirror 1 onto shielclLS at a ositron'rst'fi' left of slot 14. Thus, when mirror I is in this position, light will be impinged on photoelectric cell l3 during the intervals when plate i1 is illuminated, or in other words cell I! will be exposed to intermittent flashes of light having a frequency equal to that of the supply conductors L and IF.

As indicated in Fig. 'l by the arrow, mirror 1 is deflected clockwise from its undefiected position to a position in which light emanating from neon lamp plate I! is reflected by mirror 1 through slot l4 onto photoelectric cell II. when plate I1 is illuminated and plate It is dark, however, as shown in Fig. 6, light is reflected from plate I! by mirror 1 onto shield I! at a position to the right of slot l4. Thus, when mirror 1 is deflected clockwise from its undefiected position, photoelectric cell l3 will be exposed to intermittent flashes of light having a frequency equal to that of supply conductors L and 1? and it will be noted, the periods or phase of the flashes are opposite from those to which the cell is subjected when the mirror I is deflected in a counterclockwise direction from its undefiected position.

As will be readily apparent the emission of photoelectric cell I3 is changed in accordance with the phase of the flashes of light impinging thereon and produces across the terminals of the photoelectric cell a pulsating E. M. F. which may be readily amplified by the amplifier H and the amplified quantity is employed to selectively control the rotation of motor i2 to effect the desired rebalancing adjustments of contact it along resistors 8 and thereby restoration of the galvanometer moving coil to its normal, undefiected position. I

The means for amplifying the pulsating E. M. F.s produced across photoelectric cell It includes conductors l9 and 20 connecting the anode and photocathode, respectively, of cell [3 to the input terminals of an electronic valve 2|. Electronic valve 2| is a filament type triode and includes an anode 22, a control grid 23 and filament cathode 24 and, as shown, conductor 20 is connected to anode 22 and conductor I9 is connected to control grid 21.

Energizing voltages are supplied amplifier H SH U QU-L' and photoelectric cell l2 from a transformer 28 which is a combination step-up and step-down transformer having a primary winding 22 connected between alternating supply conductors L and L, and having two high voltage windings 21 and 22 and two low voltage windings 22 and 22. Low voltage secondary winding 22 supplies current through conductors 2| and 22 to the filament cathode 24 of valve -2| and low voltage secondary winding 2| supplies current through conductors 22 and 24 to the filament cathode 22 of a rectifier valve 25. Rectifier valve 26 may be, andis shown as, of standard type and includes a pair of anodes 21 and 22 in addition to the filament cathode 22. One terminal of the high voltage transformer secondary winding 21 is connected by a conductor 22 to the anode 21 and a terminal of the high voltage winding 28 of opposite polarity is connected by a conductor to the anode 22. Thus, the potentials of anodes 21 and 28 will be swung alternately positively and negatively at a frequency equal to that of the supply line frequency, one anode being positive while the other is negative.

The other terminal of the high voltage winding 21 is connected by a conductor 4| to one end of a motor winding I21" and the other terminal of high voltage winding 22 is connected by a conductor 42 to one end of a motor winding |2R.

Motor windings |2F and |2R are opposed windlugs and when selectively energized cause rotation of motor |2 in a forward and a reverse direction, respectively. The other ends of windings I21 and |2R. are connected together and are connected by a conductor 2 to a center tap on a resistor 44 which is connected in shunt to the filament cathode 24 of valve 2|. Anode 22 of valve 2| is connected by a conductor 45 to a center tap on a resistor 46 connected in shunt with the filament cathode 26 of rectifier valve 2!.

The motor l2 may be of the well-known capacitor induction variety having a main winding |2C continuously energized from the supply conductors L ond I? through a condenser |2D of suitable value, a squirrel cage rotor |2E. the opposed windings |2F and |2R, and a core (not shown), common to all the windings. The winding I20 and the opposed windings I21" and |2R are spaced in quadrature and so wound on the core that when the winding i21 is energized, a rotating field will be set up in the squirrel cage. rotor |2E in one direction and when the winding |2R is energized, a rotating field will be set up in the rotor in the opposite direction thereby actuating the rotor |2E for rotation in a corresponding direction.

It is noted that full wave rectified current is supplied valve 2| due to rectifying action of valve 25, but that only half wave rectified current can pass through windings I2! and |2R of motor l2. This is brought about since valve 2| is energized during one half cycle ofthe supply conductor voltage by the transformer secondary winding 21, during which half cycle the motor winding i2! is energized, and during the second half cycle of the supply conductor voltage, valve 2| is energized by the secondary winding 22, during which half cycle, the motor winding |2R will be energized.

As previously noted, the anode and photocathode of photoelectric cell |2 are connected to the anode 22 and control grid 22, respectively, of valve 2|, and as shown, a resistor 41 of suitable -value is desirably connected between the control grid 22 and the filament cathode 24. A suitable battery 22A may be provided for supplying negative bias to the control grid 22. It will be seen fiected position.

that the connection of the photoelectrc cell l2 and resistor 41 forms a circuit which shunts the anode 22 and cathode 2 of valve 2| whereby photoelectric cell I2 is energized from the same voltage source as valve 2|.

I contemplate two modes of operation of motor l2 in the system illustrated in P18. 1. In accordance with one method, the voltage of battery 22A is so choun that the normal current conducted by valve 2| is an appreciable value whereby the motor windings HF and |2R are alternately energized and motor I2 is actuated for rotation in opposite directions in rapid alternation and will remain stationary. This is the case when galvanometer I is in its normal, undefiected position. when the galvanometer deflects in one direction or the other from its normal position, in a counterclockwise direction, for example, photoelectric cell II will be exposed to the entermittent flashes of light from neon lamp plate i1 and a positive potential will be applied to control grid 22 during the half cycles when anode 21 of rectifier valve 25 control grid 22 during the half cycles when anode 22 of valve 25 is positive. The average value of current passing through motor winding |2R will then predominate over that passing through motor winding |2R and motor l2 will be actuated for rotation in the reverse direction. In accordance with the second mode of opera- .tion which I contemplate, the voltage of battery 22A is so chosen that the normal current conducted by valve 2| is a negligible value whereby the motor windings HF and |2R will both be deenergized when the galvanometer 6 is in its unde- Upon deflection of the galvanometer in one direction or the other, one or the other of windings |2F and |2R will be energized and motor |2 will be actuated for rotation in a corresponding direction.

As illustrated in Fig. 1, the shaft of motor i2 is connected in any convenient manner to a shaft 42, which may desirably be a rod having a spiral groove thereon, and the potentiometer contact I 0 is mounted on a carriage carried by shaft 42 and is adapted to be moved in one direction or the other as the shaft 42 is rotated. Thus, when motor 12 is actuated for rotation as a result of galvanometer deflection, the motor eflects an adjusting movement of the contact ill along the l-nrumnpv slidewlre resistors I in the proper direction until to cooperate with a recorder chart 4! and to thereby provide a continuous record of the temperature'of the interior of furnace l. The recorder chart is adapted to be driven by a continuously rotating roller 20 and the latter is driven in any convenient manner, as for example, by a unidirectional motor 5| through suitable gearing (not shown), so fllat a record of the temperature of the interior of furnace I will be had as a continuous line on the chart.

In Figs. 8-10 I have illustrated in detail, more or less diagrammatically, a modification oi the arrangement illustrated in detail in Figs. 2-7 for shielding photoelectric cell II from both plates of neon lamp it when the galvanometer is in its undefleeted position and for selectively exposing the cell to light from one or the other of the plates when the galvanometer deflects in one direction or the other from that position. In this modification shields 52 and 53 are positioned directly in front of neon lamp it in such manner that diametrically opposed halves of plates i1 and M are shielded. Similar shiel 54 and are positioned directly in front Qf BP O ec cell II in such manner 'th'atthe photocathode of the latter is divided into four equal rectangles, diametrically opposed rectangles being shielded by parts 54 and 55 and the remaining rectangles unshielded and arranged to be selectively exposed to the corresponding unshielded sections of neon lamp i6.

As illustrated in F1 8 which mirror I is shown in its undeflected position, it will be seen that the quarter moons reflected by the mirror toward cell It from plates. i1 and I! fall upon the respective shields N and 55 and in this position of mirror I, cell II will remain dark. In Fig. 9 mirror 1 is shown partially deflected in a countercloclrwise direction, in which position it will be noted that light irom plate I1 is reflected onto cell It. The intermittent flashes of light thus directed onto cell I! will operate to a of otor ii in a forward direction in the manner already described in connection with Fig. 1. In Fig. 10 mirror 1 is shown partially deflected in a clockwise direction, in which position light from plate ll is reflected onto cell is, and it will be apparent motor I! will then be actuated for rotation in a reverse direction.

It is noted that in this modification that the amplitude of the E. M. F. induced in photoelectric cell It will be proportional to the deflection of mirror I. As shown in Fig. 9, for example, mirror I is only deflected slightly from its normal po resl'ilt onlyaportion of the light from neon lamp plate i1 is directed on cell it. but it will be apparent that upon greater deflection of mirror 1 that more of the light from plate [1 will fall on the cell. The emisivity of cell It will then be proportional to the deflection of mirror 1 and thereby the amplitude of the energizing current to motor winding l2! and consequently the speed of rotation of the latter will be proportional to the deflection of mirror 1.

In order that the speed of motor I! may be as great as possible during rebalancing without overshooting of the balance point and consequent hunting taking place, means may be provided to ensure that the motor speed and the rate of change of its speed is substantially proportional to the extent of unbalance. Such means are desirable unless the motor has a suitable damping characteristic of its own and may be embodied in the devices shown in Figs. 11 and 12, for example. In Fig. 11, a disc It, which may be of copper, is fastened to the shaft of motor I! for rotation within a magnetic field as provided. for example, by a permanent magnet 51. Magnet 81 is carried at one end of a vane 58, and a shield Ii which may be integral with vane 58 is carried at the other end of the latter and amused in cooperative relation with photoelectric cell l8.

a shaft I and the latter is pivoted for rotation in suitable bearings (not shown). When disc ii is stationary and the galvanometer mirror 1 is in its undeflected position, vane it assumes a center position with respect to cell I! such that shield Ii blocks all light from neon lamp Ii from the cell. If desired, a suitable spring member may be provided for normally holding vane 58 in this central position.

As will be clear, upon deflection of mirror I, light from either neon lamp plate ll or it will pass through slot H of shield l and will cause motor l2 to be energized for rotation in a corresponding direction. Magnetic reaction between magnet 51 and disc 56 rotating between the poles of magnet 51 will then exert a drag on the latter which is in the proper direction to move shield IS in the direction to intercept the light falling on cell ii.

If the potentiometric unbalance is slight and thereby the deflection of mirror 1 from its neutral position is slight, the beam will be entirely intercepted by shield l5 and motor l2 will be deenergized. The resulting decrease in motor speed will cause a corresponding reduction in the drag exerted on vane 58 by disc 55 and the vane will return toward its normal central position. If the motor rotation has not been suflicient to rebalance the potentiometric network 5, mirror 1 will still be deflected from its normal position and accordingly motor l2 will again be energized for rotation. The subsequent deflection of vane 68 from its normal central position will again operate to shield photoelectric cell 13 from neon lamp l6 and thereby to cause deenergization of motor it. Thus; when the potentiometric network 5 is slightly unbalanced. the energizing current flow to motor I! which is causing rotation 01 the latter will intermittently be cut oil whereby the motor will gradually slow Vane II is carried by down and the potentiometer contact will come to rest at the exact balance point.

As will be clear, if the system is appreciably unbalanced, mirror I will be deflected to a greater extent and upon deflection of vane 58, the light to photoelectric cell I! will not be cut off. It should be noted that the individual beams of light emanating from neon lamp plates ll and II have a width equal to that of the plates and that they are separated by a distance preferably slightly greater than the width of slot H. Thus, when vane 88 is in its undeflected position, the space between plates l1 and it will be deflected on slot ll and the light from the plates will be reflected on shield i8. As will be apparent the maximum deflection of mirror I may be so adiusted with respect to the maximum deflection of vane SI that when the mirror is deflected to its fullest extent, photoelectric cell I! will still be exposed to light from neon lamp l6 whereby motor II will be continuously energized.

It will thus be clear that on an appreciable change in temperature in the interior of furnace l, motor I! will be continuously energized and eflect rapid adjustment of potentiometer contact Ill along resistors until the thermocouple E. M. I". is almost balanced, and as the balance point is approached, the energization of motor I! will be intermittently interrupted as described hereinbefore, and the speed thereof will be reduced and contact II will come to rest at the exact balance point.

In Fig. 12 a vane I which may be similar to the correspondingly identifled part of Fig. 11 is pivoted for rotation on a shaft I and a blade Ii integral with vane II is arranged in cooperative relation with a fan '2 which is carried by the shaft of motor II. It will be clear that when 5 motor i2 is energized for rotation, fan 8! will exert a drag on blade I and thereby cause deflection of vane II from its central position.

Although the devices illustrated in Figs. 11 and 12 for damping motor II have been described 10 in connection with the arrangement shown in Fig. 1, and shown in detail in Figs. 2-7 for exposing cell II to one of neon lamp plates l'l or it upon deflection of mirror I, it will be readily apparent that these motor damping devices may 15 be employed with equal facility with the arrangement disclosed in Figs. 8-10. 7

It will be apparent that motor I2 may be employed to operate a control valve or a rheostat for governing the application of an agent to a furnace for producing heat, for example to which the thermocouple 2 is responsive, or another motor desirably operated together with motor I! may be so employed. For example, as shown in Fig. 13, a furnace I, to the temperature of which thermocouple I is responsive, is heated by a resistor I which is connected to electric supply conductors LP and L through a rheostat 64, the adjustment of which is efl'ected by a motor I. The motor I may be exactly like motor I! so and is shown connected in parallel therewith. The mechanical connection of the rheostat M to the motor 65 is such as to decrease and increase the supply of electric current to resistor I as the temperature of furnace I rises above as or drops below a predetermined level.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of my invention now known to me, it will beiapparent to those skilled in the art that as changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that certain features of my invention may sometimes be used to advantage without a I corresponding use of other features.

' Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. In combination means for deriving from I an original alternating voltage source an additional alternating voltage variable in phase in accordance with the deflection of a controlling element from a predetermined position, including a light sensitive device, means under control of said controlling element to illuminate said light sensitive device with light pulsating at the frequency of said original source, and of one phase or of opposite phase accordingly as said controlling element is deflected in one direction or the other from a normal undeflected position. means under control of said light sensitive device adapted to restore the said controlling element to said undeflected position on departure therefrom, and including a shield adapted to intercept iightfrom said illuminating means ofone phase, or of opposite phase when said controlling element is deflected in one direction or the other from said undeflected position, and means to is temporarily shift said shield while said controlling element is deflected tosimulate restoration of said controlling element to its undeflected position and thereby arrest the operation of said restoring means.

2. In combination means for deriving from an original alternating voltage source an additional alternating voltage variable in phase in accord- 5 ance with the deflection of a controlling element from a predetermined position including a light sensitive device, means under control of said controlling element to illuminate said light sensitive device with light pulsating at the frequency of said original source and of one phase or of opposite phase accordingly as said controlling element is deflected in one direction or the other from a normal undeflected position, means shielding said light sensitive device from said illuminating means when said controlling element is in a normal undeflected position, means under control of said light sensitive device adapted to restore the said controlling element to said undeflected position on departure therefrom, and

, means to temporarily shift said shielding means while said controlling element is deflected to simulate restoration of said element to its undeflected position and thereby arrest the operation of said restoring means.

3. In combination means for deriving from an original alternating voltage source an additional alternating voltage variable in phase in accordance with the deflection of a controlling element from a predetermined position, including at least two independent light emitting elements energized on alternating half-cycles, one of said light emitting elements being energized while the other is de-energized, a light sensitive device. means for shielding said light sensitive-device from both of said light emitting elements when said controlling element is deflected in one direction or the other from said normal undeflected position, means under control of said light sensitive device adapted to restore the said controlling element to 4 said undeflected position on departure therefrom, and means to temporarily shift said shielding means toward a position wherein the latter intercepts the light to said light sensitive cell to anticipate the return of said controlling element to said imdeflected position.

4. In combination means for deriving from an original alternating voltage source an additional alternating voltage variable in phase in accordance with the deflection of a controlling element 80 from a predetermined position, including at least two independent light emitting elements energised on alternating half-cycles, one of said light emitting elements being energised while the other is de-energised. a light sensitive device. means for shielding said light sensitive device from both .of. said light emitting elements when said controlling element is in. a .normal undeflected position but adapted to expose said light sensitive device to one or the other of said light emitting elements when said controlling element is deflected in one direction or the other from said normal undeflected position, means under control of said light sensitive device adapted to restore the said controlling element to said undeflected position 05 on departure therefrom, 'and means to arrest the operation of said restoring means including a device responsive to the operation of said lastmentioned means and adapted to shift said shieldingmeansinthedirectiontoreducethe'lo effect of deflection of said control element.

. THOMAS R. HARRISON. 

